JPS5820546B2 - Protection circuit for switched mode power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention extracts DC output power of a required voltage value from an inductance element whose opening and closing of power supply from a DC power supply is controlled by at least one switching element whose opening and closing are controlled by a switching pulse. Further, the variation in the voltage value of the DC output power taken out from the inductance element is detected, and the impact coefficient in the switching pulse is changed in accordance with the variation in the voltage value. In a switched mode power supply circuit where the voltage value of the DC output power taken out from the inductance element is controlled so that it remains constant, the switching element Protection of switched mode power supply circuits that have excellent features such as good protection, no malfunctions, reliable detection of abnormal conditions, and automatic recovery function. It was created for the purpose of providing a circuit.

First, with reference to FIGS. 1 and 2, the structure, operation, and problems of a conventional protection circuit for a switched mode power supply circuit will be explained.

FIG. 1 is a block diagram showing one of the conventional examples of a switched mode power supply circuit equipped with a protection circuit, and in this FIG. After rectification and filtering, output voltage V between output terminals 7 and 8
This is a DC power supply that generates DC power of i, and this DC power supply 1 has four rectifying elements (diodes) in the illustrated example.
It is shown as being constituted by a bridge-type full-wave rectifier circuit consisting of circuits 2 to 5 and a smoothing capacitor 6.

A series connection circuit of an inductance element 9, a switching element 13, and a resistor 14 is connected to the output terminals 7 and 8 of the DC power supply 1, and when the switching element 13 performs a switching operation, the inductance element is removed from the DC power supply 1. The DC power supplied to the inductance element 9 is stored as magnetic energy or taken out from the inductance element 9 as electric power.

In the illustrated example, a transformer 9 consisting of a primary winding 1o, a secondary winding 11, and a tertiary winding 12 is used as the inductance element 9, but a choke coil may also be used as the inductance element 9. good.

Further, in the illustrated example, one transistor 13 is used as the switching element 13, but a plurality of transistors or one or more ground-type active elements may be used.

The resistor 14 connected to the emitter of the transistor 13 is
A resistor for detecting the current flowing through the transistor 13,
The operation and problems of this part will be discussed later.

In the figure, the black circles attached to each winding 10 to 12 of the transformer 9 used as the inductance element 9 indicate the voltage polarity when the transistor 13 is in the on state. Energy is stored when the transistor 13 is in the on state, and the stored energy is released from the secondary winding 11 and the tertiary winding 12 to the subsequent circuit when the transistor 13 is in the off state.

The secondary winding 11 of the transformer 9 is provided with a rectifying and smoothing circuit consisting of a rectifying element 15 and a smoothing capacitor 16, and the tertiary winding 12 of the transformer 9 is provided with a rectifying element 3o and a smoothing capacitor 31. A rectifying and smoothing circuit is provided, and the DC power of the DC voltage Vo obtained in the circuit on the one side of the secondary winding 11 is supplied to the load 19 from the output terminal ・17. The DC output obtained by the circuit on the tertiary winding 12 side is given to an error voltage detection circuit, which will be described later, via an output terminal 28.

The block 27 is a switching pulse generation circuit that is supplied to the base of the transistor 13 used as the switching element 13, and in the illustrated example, the operating power in the switching pulse generation circuit 27 is
The switching pulse is supplied to the switching pulse generation circuit 27 from a small-capacity DC power supply 20.

In the switching pulse generation circuit 27, 23 is an error detection amplifier, 24 is a pulse width modulator, 25 is a sawtooth wave generator, and 26 is an amplifier.
3 is the voltage Vx obtained from the slider of the variable resistor 33 in the resistance circuit network of the error voltage detection circuit, which is composed of the resistor 32 connected to the terminal 28, the variable resistor 33, the resistor 34, etc., and the zener voltage Vx. It compares it with the reference voltage Vs set by the diode 22, and provides the pulse width modulator 24 with an error output voltage VB corresponding to the variation of the output voltage (2) 0.

Further, the pulse width modulator 24 is supplied with a sawtooth wave voltage A having a constant repetition period Ts from the sawtooth wave generator 25.
is given, the pulse width of the output pulse ~ from the pulse width modulator 24 corresponds to the period vA>VB, and therefore, the change in the error output voltage vB from the error detection amplifier 23 corresponds to The pulse width of the output pulse Pw from the pulse width modulator 24 changes as shown in FIG.

In addition, in FIG. 2a, vA is the sawtooth wave generator 2.
5 is a sawtooth voltage with a repetition period Ts, and VB is an error output voltage from the error detection amplifier 23.

The output pulse Pw from the pulse width modulator 24 is sent to the amplifier 26.
After being amplified by , it is supplied to the base of the transistor 13 as a switching pulse Ps.

Therefore, the switching pulse P supplied from the switching pulse generation circuit 27 to the base of the transistor 13 used as the switching element 13
The pulse width of s is changed so that the voltage value of the DC power applied to the load 19 is always a constant value Vo. The impact coefficient (
The amount of energy supplied from the DC power source 1 to the inductance element 9 is controlled by being turned on and off by the switching pulse Ps whose duty cycle, duty factor) changes, and the voltage value of the DC power given to the load 19 is controlled. It is always maintained at a predetermined constant voltage Vo.

That is, when the voltage value of the DC power applied to the load 19 from the terminals 17 and 18 is higher (lower) than the predetermined voltage value Vo, the voltage Vx detected by the error voltage detection circuit increases (decreases). As a result, the error output voltage vB from the error detection amplifier 23 rises (falls), so that the impulse coefficient of the output pulse Pw from the pulse width modulator 24 becomes small (dog), and the switching pulse Ps output from the amplifier 26 The impact coefficient of becomes small (dog), the conduction period of the transistor 13 becomes short (long), and the amount of energy stored in the inductance element 9 from the DC power supply 1 and released to the load side decreases (increases). Thus, the voltage value of the DC power supplied from the terminals 17, 18 to the load 19 is maintained at a constant voltage value Vo.

In the above explanation, the switching pulse generation circuit 2
7, the sawtooth wave generator 25 always generates a sawtooth voltage with a constant repetition period Ts, and the clip level for the sawtooth voltage vA is changed by the error output voltage vB from the error detection amplifier 23. Assume that a switching pulse Ps whose pulse width changes according to the magnitude of the error output voltage vB is outputted.
The switching pulse generation circuit 27 changes the repetition period of the sawtooth wave generated by the sawtooth wave generator 25 according to the error output voltage VB from the error detection amplifier 23, and changes the repetition period of the sawtooth wave generated by the sawtooth wave generator 25. Even if a switching pulse Ps whose pulse width changes according to the magnitude of the error output voltage vB is output by clipping the voltage I at a constant voltage clip level is used. It's good.

As mentioned above, the above-mentioned components of the switched mode power supply circuit shown in the first diagram are such that the switching pulse Ps supplied from the switching pulse generation circuit 27 to the base of the transistor 13 is equal to or smaller than the error output voltage vB of the error detection amplifier 23. It operates to maintain the DC voltage value of the DC power supplied from the terminal 17°18 to the load 19 at a predetermined voltage Vo by changing the magnitude of the impact coefficient according to the magnitude of the load 19. In the event of an abnormality such as a short circuit, the switching pulse Ps applied from the switching pulse generation circuit 27 to the base of the transistor 13
The impact coefficient becomes so large that the transistor 13 may be destroyed.

Therefore, in the switched mode power supply circuit shown in FIG. By applying a proportional voltage to the gate of the thyristor 21, when a large current flows through the transistor 13, the thyristor 21 is operated, and the voltage from the amplifier 26 in the switching pulse generation circuit 27 to the transistor 13 is Switching pulse Ps to base
We are trying to cut off the supply of

However, if the thyristor 21 is used in the configuration as a protection circuit as described above, once the protection circuit operates, the protection circuit will continue to operate unless the main power switch is opened and closed again. In other words, since the protection circuit in which Cyrisk is used does not have an automatic recovery function, for example, if the load size suddenly changes, the resistor 14 connected to the emitter of the transistor 13 Even in situations that do not normally need to be protected by a protection circuit, such as when a large voltage appears for a short time and quickly returns to normal, the resistor 14
However, if the protection circuit starts its protective operation due to a large voltage that momentarily appears, the protection circuit continues to perform its protective operation permanently.

The above-mentioned voltage change state in the resistor 14 due to a sudden change in load also occurs when, for example, a television receiver is used as a load and the video content suddenly changes from black to white. Even in cases where the protection circuit mistakenly starts a protection operation and the protection circuit does not perform an automatic recovery operation, this is a problem from the standpoint of the performance of the television receiver.

In order to eliminate some of the above drawbacks, if the resistor 14 connected to the emitter of the transistor 13 is made to have a low resistance value, the transistor 13
Because the detection sensitivity of the amount of current flowing through the
The problem is that the time required from the occurrence of an abnormality to the operation of the protection circuit increases, and therefore the protection function of the protection circuit for the transistor 13 becomes insufficient.

The present invention provides a protection circuit for a switched mode power supply circuit that does not have the problems described above in the conventional protection circuit for a switched mode power supply circuit. 1 shows a block diagram of embodiments; FIG.

In FIG. 6, the same reference numerals as those used in FIG. 1 are given to components that are equivalent to those in the circuit layout shown in FIG. 1 described above.

FIG. 3 (here, the block MM indicated by a broken line frame is a monostable multi-bi break, and the monostable multi-bi break MM shown in FIG. 3 has a configuration suitable for integrated circuit (IC) Although this is an example of a monostable multi-bibreak MM, other configurations may of course be used.

In the monostable multi-bi break MM, the transistors 46 in the transistors 45 and 46 provided in the constant current circuit 4T in the emitter circuit are rendered non-conductive (off) when the mono-stable multi-bi break MM is in a stable state, and The transistor 45 is rendered conductive (turned on) when the monostable multi-bibreak MM is stable.

When the transistor 46 is off, the transistors 43, 39, and 48 are all off, and the base voltage of the transistor 46 is equal to the voltage V of the power supply 57.
c to the base-emitter forward voltage drop VBE of the transistor 51 (the base-emitter forward voltage drop of other transistors is also
BE is approximately equal to BE, so in the following explanation it is assumed that the forward voltage drop between the base and emitter of any transistor is VBE).

When transistor 46 is off, transistor 45
The power supply voltage Vcc is connected to the base of the resistor 36°37.38
A voltage H obtained by dividing the voltage by a resistor network consisting of
), there is a relationship of vH>(Vc −VB E), and therefore, the transistor 45
is on, the transistor 46 remains off, and the monostable multi-bibreak MM is maintained in a stable state.

At time t1, the base of the transistor 39 is connected to the resistor 14 connected to the emitter of the transistor 13, which is used as a switching element 13 for controlling the opening and closing of power supply to the inductance element 9.

Voltage Va larger than emitter forward voltage drop VBE
When this occurs (see Figure 4a), the transistor 39 is turned on and the resistor 38 is almost short-circuited, and the power supply voltage is applied to the base of the transistor 45 between the resistors 36 and 37.
Voltage vL divided by (however, VL<(VC
VBE)) will be given, and the transistor 45
is turned off, and at the same time transistor 46 is turned on.

When transistor 46 is turned on, transistors 43 and 48 are also turned on, and transistor 39 is also turned on.
is also kept on.

The capacitor C connected between the base of the transistor 46 and the ground is discharged through the resistor R due to conduction of the transistor 48, and the terminal voltage of the capacitor C changes on the time axis as shown in FIG. .

When the terminal voltage of the capacitor C mentioned above falls below the base voltage ■L of the transistor 45 mentioned above at time t2, the transistor 46 turns off at time t2,
At the same time, transistors 43, 39, and 48 are all turned off.

By turning off the transistor 39 as described above, the voltage v is again applied to the base of the transistor 45.
Since H is applied, the transistor 45 returns from the off state to the on state.

From the point at which the transistor 46 changes from the on state to the off state and the transistor 48 also changes from the on state to the off state, the capacitor C begins to be charged by the power supply 57 via the resistor 50, and its terminal The voltage is the emitter voltage of the transistor 51 (Vc
-VBB).

The base voltage of the transistor 59 whose base is connected to the collector of the transistor 48 described above is determined by the on/off operation of each transistor in the monostable multi-bibreak MM described above and the charging/discharging of the time constant circuit by the resistor R250, capacitor C, etc. Depending on the operation or the like, it changes on the time axis as shown in FIG. 4C.

The collector of the transistor 59 is grounded,
Its emitter is connected to the emitter of a transistor 56 via a resistor 58, and the collector of the transistor 56 whose base is connected to a power supply 57 is connected to the collector of the transistor 54 and the bases of the transistors 54 and 55, Further, the emitter of the transistor 54 is connected to the power supply voltage Vcc through a resistor 52, the emitter of the transistor 55 is connected to the power supply voltage Vcc through a resistor 53, and the collector of the transistor 55 is connected to the error detection amplifier 23 and the pulse width modulator. The pulse width modulator 24 side end of the resistor 35 connected between the pulse width modulator 24 and the pulse width modulator 24 is connected to the resistor 35 .

Therefore, the voltage at the base of transistor 59 is
When it changes on the time axis as shown in Figure C, that is,
The monostable multi-bibreak MM changes from a stable state to an unstable state due to an abnormally large voltage Va generated in the resistor 14 connected to the emitter of the transistor 13.
Corresponding to the series of operations to return to a stable state again, the voltage of the time constant circuit in the monostable multi-bibreak MM changes on the time axis, so that the base voltage of the transistor 59 becomes lower than (Vc - 2VB E ). It becomes conductive over the period t1 → t3 with a voltage value of
Accordingly, the transistors 54 to 56 also become conductive over the period from t1 to t3.

The period from t1 to t3 described above is made to be sufficiently longer than the repetition period Ts of the sawtooth voltage vA (the repetition period of the switching pulse Ps is also the same).

Assuming that the input impedance of the pulse width modulator 24 is sufficiently high and the output impedance of the error detection amplifier 23 is sufficiently low, the above-mentioned transistors 59, 54 to 5
The collector current of the transistor 55 flowing during the conduction period of 6 is connected to the output side of the error detection amplifier 23 and the pulse width modulator 2.
Since the error output voltage V is applied to the pulse width modulator 24 when the transistors 59 and 54 to 56 are conductive, the error output voltage V
B/ is the sum of the original error output voltage vB output from the error detection amplifier 23 and the voltage drop across the resistor 35 due to the collector current of the transistor 55 (d in FIG. 4).
It will look like the one shown.

That is, in the period from t1 to t3 when the transistors 59, 54 to 56 are conductive, from time t1 when the transistor 48 in the monostable multi-bibreak MM is turned on to time t2 when it is turned off, Since the transistor 48 is operating in the saturation region,
The voltage at the base of the transistor 59 connected to the collector of the transistor 48 is set to approximately the ground potential, so a large current flows through the transistors 59, 54 to 56, and the voltage drop generated across the resistor 35 due to the collector current of the transistor 55 is also reduced. It becomes large as shown in the period t1→t2 in FIG. 4d.

At time t2 when the transistor 48 changes from the on state to the off state, the base voltage of the transistor 59 suddenly changes from the above-described state of approximately the ground voltage to the terminal voltage VL of the capacitor C at time t2, Thereafter, as the terminal voltage of the capacitor C increases as the capacitor C is charged through the resistor 50, it increases toward the voltage value (Vc-VBB) as shown in the curve shown in FIG. 59. The amount of current flowing through 54 to 56 decreases as shown in the curve shown in FIG. t of
It gradually decreases as shown in the period 2→t3.

Therefore, at time t2 when the transistor 48 changes from the on state to the off state, vL suddenly changes from the ground voltage.
The voltage drop Vα generated across the resistor 35 by the collector current flowing to the collector of the transistor 55 when the base voltage of the transistor 59 has increased to a voltage value of If the value is set to be sufficiently larger than the peak value of the sawtooth voltage VA, the resistor 14 connected to the emitter of the transistor 13 will be connected to the When a large voltage Va as shown in the figure is generated and the monostable multi-bibreak MM is activated, the entire period from time t1 to t2 and time t
2→t3, the voltage value of the error output voltage vBl applied to the pulse modulator 24 is higher than the peak value of the sawtooth wave voltage vA supplied to the pulse modulator 24. Since the pulse modulator 24 has a voltage value, no pulse is sent out from the pulse modulator 24, and therefore, the switching pulse Ps can be prevented from being supplied to the switching element 13 from the amplifier 26 of the switching pulse generation circuit 27.

Next, in the period from time t2 to t3, the transistor 5
The base voltage of 9 changes from the voltage value vL to the voltage value (Vc-2VB
In the process, the collector current of the transistor 55 also gradually decreases, and as the voltage drop across the resistor 35 due to the collector current of the transistor 55 also gradually decreases, the pulse The voltage value of the error output voltage VB' applied to the width modulator 24 also gradually decreases, reaching a voltage value that matches the peak value of the sawtooth wave voltage vA, and then forever reaching a voltage value lower than the peak value of the sawtooth wave voltage ■A. do.

As the voltage value of the error output voltage VB' applied to the pulse width modulator 24 becomes lower than the peak value of the sawtooth voltage VA, the level of the error output voltage VB' from the pulse modulator 24 decreases. Pulses Pw whose impact coefficients gradually become larger in accordance with are sequentially sent out, which are amplified by the amplifier 26 and applied to the switching element 13 as switching pulses Ps.

As described above, in the protection circuit of the switched mode power supply circuit of the present invention, upon detecting an increase in the voltage drop occurring across the resistor 14, the protection circuit operates immediately and stops supplying the switching pulse Ps to the switching element 13. death,
The switching element 13 can be completely protected.

The protection circuit automatically returns after a certain period of time after it starts operating, and the switching pulse Ps whose impact coefficient gradually increases on the time axis is supplied to the switching element 13. It controls the switching pulse generation circuit 27.

In this way, the switching element 13 after automatic recovery
The fact that the switching pulse Ps supplied to the switching element 13 is gradually changed from one with an impact coefficient of O to one with a larger impact coefficient ensures complete protection of the switching element 13 at the time of automatic return. This is highly desirable since it can be carried out in many cases.

Now, due to some reason, a large voltage drop occurs across the resistor 14, causing the protection circuit to start operating as described above, and then automatically recover after a certain period of time has elapsed, causing the impact coefficient on the switching element 13 to gradually increase. If the load remains short-circuited when the switching pulse Ps is supplied, a large voltage drop will occur across the resistor 14 again, so the protection circuit will start operating again and immediately stop supplying the switching pulse Ps. is stopped, and the switching element 13 is completely protected.

As long as the load is short-circuited, the protection circuit repeats operation → automatic recovery → operation → automatic recovery, etc. However, since excessive current will not flow through the transistor 13 used as a switching element, Transistor 13 is now completely protected.

The protection circuit of the switched mode power supply circuit of the present invention has a third protection circuit.
It goes without saying that the present invention can also be effectively applied to switched mode power supply circuits having configurations other than the configuration shown in the figure.

As is clear from the detailed explanation above, the protection circuit for the switched mode power supply circuit of the present invention is such that the magnitude of the current flowing through the switching element that controls opening and closing of the power supply to the inductance element is set to a predetermined value. If it exceeds the limit, the monostable multi-bi break shifts from a stable state to an unstable state and the supply of switching pulses to the switching element is immediately stopped. The switching element is protected from excessive current by changing the impulse coefficient of the switching pulse to be supplied to the switching element so as to gradually increase from 0 on the time axis based on the voltage that changes. Since it has a reset function, the switching element is completely protected and does not malfunction, and furthermore, the circuit for detecting the current flowing through the switching element can be easily configured. According to the present invention, it is possible to easily provide a switched mode power supply circuit with excellent performance in which all the drawbacks of the conventional ones mentioned above are satisfactorily eliminated.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional circuit, and FIG. 2a is a block diagram of a conventional circuit. FIG. b and FIG. 4 a = d are explanatory waveform diagrams, and FIG. 3 is a block diagram of one embodiment of the protection circuit for the switched mode power supply circuit of the present invention. 0... AC power supply, 1... DC power supply, 9... Inductance element, 13... Switching element, 22...
・Zena diode, 23...Error detection amplifier, 2
4... Pulse width modulator, 25... Sawtooth wave generator, 39°43.45, 46, 48, 54-56.59.
...Transistor, 14, 32.34~3B, 40~
42.44,50,52,53,58. R...Resistance,
C... Capacitor, MM... Monostable multi-by-break, 20.57... Power supply.

Claims (1)

[Claims] 1. A DC power supply that rectifies and smoothes AC power to generate DC power; an inductance element whose opening/closing of power supply from the DC power supply is controlled by at least one switching element whose opening/closing is controlled by a switching pulse; means for detecting fluctuations in the voltage value of DC output power obtained by rectifying and smoothing the voltage generated in the inductance element; means for changing the impact coefficient of the switching pulse according to the fluctuations in the voltage value; A switched device is equipped with means for detecting a voltage according to the current value flowing through the element, and is controlled so that the voltage value of the DC output power taken out from the inductance element is constant. A protection circuit for a mode power supply circuit, comprising means for detecting a voltage according to a current value flowing through a switching element whose opening and closing are controlled by a switching pulse, and when the current value flowing through the switching element reaches a predetermined value. A monostable multivibrator configured to transition from a stable state to an unstable state based on the output from a means for detecting a voltage corresponding to the current value flowing through the switching element, and the monostable multivibrator described above. means for generating a voltage whose magnitude changes on the time axis in a time constant circuit in a monostable multibibreak when the voltage is in an unstable state; means for changing the impulse coefficient of the switching pulse to be supplied to the switching pulse such that the impulse coefficient of the switching pulse is O
a protection circuit for a switched mode power supply circuit, comprising: means for generating switching pulses increasing gradually from a state of .